The present invention relates to a polyamide film production method which ensures stable high-speed production of a stretched polyamide film and, particularly, to a polyamide film production method which is characterized by a molten sheet cooling and solidifying process to be performed after a molten polyamide resin is extruded from a die.
In production of an unstretched sheet of a polyamide resin by a T-die method, a molten sheet extruded from a die is pressed against the surface of a rotary cooling roll (CR) called xe2x80x9ccasting rollxe2x80x9d for cooling and solidification thereof.
For the pressing of the molten sheet against the CR, air is evenly blown along the width of the molten sheet by means of an air knife apparatus (hereinafter referred to as xe2x80x9cair knife methodxe2x80x9d). Alternatively, an electrostatic adhesion method is employed in which electric charges are generated on the molten sheet by a high voltage electrode to cause the molten sheet to electrostatically adhere on the CR.
In the air knife method, adhesion between the molten sheet and the CR is weak. Therefore, the air knife method cannot be applied to a resin such as a polyester resin which undergoes a greater volume reduction during solidification thereof with a greater shrinkage factor, because a sheet of such a resin is liable to be dislodged from the surface of the CR. On the contrary, the air knife method can be applied to the polyamide resin.
In the air knife method, an air stream pressure associated with the rotation of the CR and a component of a melt tension are exerted on points of contact between the molten sheet and the CR, so that a lifting force acts on the molten sheet. Air is blown over the molten sheet on the CR surface to apply an air pressure against the lifting force by the air knife. However, a very small amount of air is trapped between the molten sheet and the CR thereby to form a thin air layer therebetween. In general, a sheet of a high-crystallinity polyamide resin is cooled at different cooling rates and, hence, has different crystallinities depending on the thickness of the air layer. The uniformity and stability of the crystallinity of the sheet are most important for controlling the stretching performance and operability of a high-speed stretched film production line.
Where a sheet production speed is relatively low, the air layer has a uniform thickness with little variation along the width of the sheet. Where the sheet production speed is increased, however, there is a disadvantage such that the air layer is not uniformly formed between the molten sheet and the CR surface. Since air is nonuniformly trapped between the molten sheet and the CR surface, there are greater variations in the thickness of the air layer, resulting in formation of spots. This phenomenon is particularly remarkable in edge portions of the sheet.
The spots formed in the edge portions of the sheet seriously hinder high-speed sheet feeding in the subsequent process. Particularly in a water absorption process, spots swelled by water absorption in the unstretched sheet hinder the sheet from passing straight in a hot water bath. This causes the sheet to meander and wrinkle, so that the sheet cannot properly be subjected to a water immersing process. As a result, high-speed production cannot be achieved.
Further, the spots formed in the edge portions of the sheet affect the stretching of the sheet in a stretching process. Where the crystallinity of the edge portions of the sheet is excessively low, the sheet is liable to be broken around the edge portions thereof when the stretching process is performed with the edge portions being held by tenter clips. This results in an increase in breakage frequency, which is a serious problem for the production process. This reduces the yield of the product film, and deteriorates the physical properties (e.g., thickness uniformity, surface smoothness and shrinkage) of the resulting stretched film.
Where the electrostatic adhesion method is applied to the polyamide resin, electric charges are transferred to the T-die and the CR because the molten polyamide resin has a high electric conductivity. In the high-speed sheet production, high-amperage electric discharge is required for the molten sheet to have residual electric charges sufficient to generate a Coulomb force for adhesion to the CR. However, the electrode has a limited electric discharge capacity, so that the electrostatic adhesion method poses a limitation to the sheet production speed.
Where the electrostatic adhesion method is employed for the production of the polyamide resin sheet, the breakage of the sheet frequently occurs in the stretching process. The cause of the sheet breakage is not clarified, but the sheet breakage is supposedly caused by a damage to the sheet due to local electric discharge, and by unstable water absorption and stretching behavior attributable to an extremely low crystallinity of the sheet due to abrupt cooling of the molten sheet on the CR without the intervention of an air layer between the sheet and the CR.
As described above, the prior art fails to provide a satisfactory film production method which ensures industrially stable high-speed production of a stretched polyamide film.
It is an object of the present invention to provide a film production method in which an unstretched polyamide sheet having a moderate and uniform crystallinity is produced to ensure stable high-speed sheet feeding in the subsequent process thereby to industrially produce a stretched polyamide film having uniform physical properties at a high speed with a high productivity.
As a result of intensive studies for achieving the aforesaid object, the inventors of the present invention have found that an unstretched sheet having a moderate and uniform crystallinity can be produced by controlling the widthwise distribution of the average thickness of an air layer present between a CR surface and a molten sheet in the air knife method, whereby a stretched polyamide film having uniform physical properties can stably be produced at a high speed.
In accordance with the present invention, there is provided a polyamide film production method, which comprises the steps of: extruding a molten polyamide resin from a die into a sheet form on a rotary cooling roll having a roughened surface; pressing the sheet against the surface of the cooling roll with the intervention of an air layer between the cooling roll and the sheet by blowing air on the sheet from an air knife apparatus for cooling the sheet, the air layer having a widthwise thickness distribution such that an average air layer thickness Te in lateral edge regions of the sheet is greater than an average air layer thickness Tc in a middle region of the sheet; and biaxially stretching the sheet.